The present invention relates to a constant current battery charger system for portable apparatuses, particularly for cellular telephones, which permits a normal use of the apparatus also during recharging of its internal battery, having a simplified architecture without auxiliary power supply circuits for powering the functional circuits of the apparatus during recharging.
Many portable apparatuses and in particular cellular telephones employ rechargeable batteries for example NiCd or Al/NiCd or Al/NiMH batteries. These internal batteries require frequent recharging because a fully charged battery is normally discharged in about an hour of continuous use of the apparatus.
Commonly, battery charger systems that do not contemplate the possibility of using the portable apparatus during recharging of its internal battery have an architecture as shown in FIG. 1.
Typically, the internal battery of the apparatus is recharged at constant current (Icharge) delivered by a suitable power supply. During recharging, two parameters are continuously monitored, namely: the battery voltage and temperature. Temperature is commonly detected by a negative, temperature coefficient resistance (NTC), purposely inserted in the battery pack. By measuring these two parameters it is possible to determine when the battery has reached full charge and automatically stop the charging in order to prevent damages to the battery that may derive from an excessive overcharge.
Under overcharge conditions, the battery voltage tends to drop from a maximum value reached during recharging (-.DELTA.V) and the temperature of the battery pack tends to raise rapidly with a certain gradient (.DELTA.T/.DELTA.t). Voltage and temperature, are measured by analog-digital converters and the digital information is processed by a supervising microprocessor. When the values of -.DELTA.V and .DELTA.T/.DELTA.t reach a preset level, the microprocessor intervenes on the constant current power supply circuit of the charger for reducing the output current or for arresting it completely.
The availability of a microprocessor further permits, by using an internal timer, to stop the charging process after a set period of time, in order to eliminate risks to the battery in case of malfunctions.
The above described charger system is satisfactory if the internal battery is preventively isolated from its load circuit (for example from the functional circuits of the portable telephone powered by the battery) during recharging. This implies that during the charging period the apparatus is unusable. In view of the fact that the recharging time may be in the order of two hours (depending on the capacity of the internal battery and of the charging current level), the inability of using the telephone or the portable apparatus during such a long period represents a drawback. The availability of a second (reserve) battery pack is a costly and/or unpractical solution.
On the other hand, if the telephone or other portable apparatus is put in operation during recharging, the current absorbed by the functional circuits subtracts from the charging current of the battery provided by the constant current battery charger. Therefore, only a portion of a markedly lower value remain available for continuing the charging of the battery. Thus, the maximum battery charge period, set by the internal timer of the charging system may expire before the battery is completely charged. If the current absorbed by the apparatus is larger than the charging current delivered by the constant current battery charger system, the battery under charge must provide the current difference, thus discharging also during the period of recharging.
Of course, such a drawback may be overcome by employing a known battery charger system, purposely modified, as schematically shown in FIG. 2.
According to this type of battery charger, in order to permit the use of the portable apparatus (e.g. a cellular telephone) during a recharging of its internal battery, without incurring the above-noted problems, the battery charger system may be provided with a dedicated auxiliary power supply circuit, capable of suitably power the apparatus at a voltage slightly higher than the battery voltage during a recharging phase of the internal battery, without jeopardizing the charging process. A fourth connecting wire of the portable apparatus to the battery charger, permits to connect to the supply node A of the functional circuits of the portable apparatus an auxiliary power supply, the output voltage of which (Vsupply) may have a value such as to always be higher than the battery voltage V.sub.B.
A decoupling element (for example a diode D) between the battery rail V.sub.B and the supply node A, ensures that the current absorbed by the apparatus (telephone) during a recharging process of its internal battery, be exclusively provided by the dedicated line Vsupply, in view of the fact that the diode D is reverse-biased.
When the apparatus is off, the diode D will not permit to the battery being charged to absorb current from the auxiliary supply line Vsupply, so that, whichever the condition of the apparatus (on or off), the charge current of the battery is always and only the constant current Icharge. Of course, the decoupling element, which in the depicted example is constituted by the diode D, may also be of other type. For example, the decoupling may be realized by mechanical means (for example a microswitch or jack connector) that are automatically acted upon when connecting the portable apparatus to the battery charger.
Therefore, the telephone or apparatus may be safely used also during the recharging of its internal battery.
On the other hand, it is evident that the addition of a dedicated auxiliary power supply circuit in a constant current battery charger system implies an enlargement of the electronic part of the system which is normally realized in integrated form, and an accompanying increase of the costs.
It has now been found and constitutes the subject matter of the present invention an outstandingly economical way of providing an auxiliary power supply output at an appropriate voltage for the functional circuits of the portable apparatus, during a recharging process of its internal battery.
Basically, the system of the invention is capable of deriving an output line at a voltage that is essentially higher than the voltage present across the terminals of the battery being charged, by deriving such an auxiliary supply line before a sensing resistance that is functionally connected in series to the path of the charging current of the battery, that is between an inductor and the output node of a converter circuit of the battery charger. On the other hand, the sensing resistance constitutes the element that provides an information on the current output by the battery charger to a feedback loop that stabilizes the charge current delivered to the battery.
In a constant current battery charger system of the type considered, the loop that regulates the output voltage of the battery charger is normally inactive because, in view of the fact that the load is a battery, the load battery itself fixes the voltage, practically preventing the output voltage of the battery charger from raising to a level such as to activate the voltage regulating loop. In practice, the voltage regulating loop would be activated only if the battery is disconnected from the battery charger terminals, but of course this is an unimportant contingency in the particular context.